Obstructive sleep apnoea and cardiovascular disease Manuel Sánchez-de-la-Torre, Francisco Campos-Rodriguez, Ferran Barbé

Obstructive sleep apnoea (OSA) is a common health concern caused by repeated episodes of collapse of the upper airway during sleep. The events associated with OSA lead to brain arousal, intrathoracic pressure changes, and intermittent episodes of hypoxaemia and reoxygenation. These events activate pathways such as oxidative stress, sympathetic activation, inflammation, hypercoagulability, endothelial dysfunction, and metabolic dysregulation that predispose patients with OSA to hypertension and atherosclerosis. OSA is a common cause of systemic hypertension and should be suspected in hypertensive individuals, especially those with resistant hypertension. In patients with OSA, continuous positive airway pressure (CPAP) treatment reduces blood pressure, and its effects are related to compliance and baseline blood pressure. Evidence suggests that OSA is a risk factor for stroke and heart failure. An association between coronary heart disease and OSA seems to be limited to middle-aged men (30–70 years). Cardiac rhythm disorders occur in about half of patients with OSA, but their clinical relevance is still unknown. The association of OSA with cardiovascular risk is mainly based on studies in men, and an association has yet to be established in women. Data on older patients is similarly scarce. Currently, there is not enough evidence to support treatment with CPAP for primary or secondary prevention of cardiovascular disease.


Oxidative stress

Obstructive sleep apnoea (OSA) is a common disease that affects 3–7% of the middle-aged population (30–70 years) and becomes more prevalent with age.1 It is caused by intermittent collapse of the upper airway during sleep, which leads to transient asphyxia. Although OSA can be asymptomatic, clinically it is characterised by intermittent snoring, unrefreshing sleep, and daytime sleepiness. OSA is an important public health issue, because it is associated with development of cardiovascular events, negative effect on quality of life, and has a causative role in traffic accidents. Only around 10% of individuals with OSA are diagnosed and treated. This shortfall has direct consequences on public health because of the high financial costs of untreated OSA. The events associated with collapse of the upper airway lead to brain arousal, changes in intrathoracic pressure, and intermittent episodes of hypoxaemia and reoxygenation. These events take place in repetitive cycles during sleep and induce the activation of various pathways (intermediate mechanisms) that predispose to atherosclerosis. Basic research and epidemiological and clinical data support the notion that OSA has a role in the initiation or progression of several cardiovascular diseases. In this Review, we describe mechanisms by which OSA might contribute to pathogenesis of cardiovascular disease, and assess clinical and epidemiological evidence of such an association.

Oxidative stress arises from an imbalance in redox status between the production and removal of reactive oxygen species (ROS). ROS can react with and cause damage to lipids, proteins, and nucleic acids, which is the pathogenic basis of age-related and chronic diseases such as cancer, cardiovascular disease, diabetes, chronic inflammation, and neurodegenerative disorders. The hypothesis that intermittent hypoxia causes oxidative stress stems from observations that hypoxia and reoxygenation injury triggers an increase in ROS production, mainly during restoration of tissue oxygenation.2,3 Non-randomised controlled trials in patients with OSA have shown increases in ROS production in monocyte and granulocyte subpopulations,4 isoprostane concentration in exhaled condensate,5 and serum malondialdehyde6 and neutrophil superoxide production.7 A randomised trial showed that patients with OSA had significantly higher isoprostane and lower nitric oxide concentrations than healthy individuals, and that these were normalised by CPAP therapy.8 In addition to an increase in ROS production, some researchers have suggested that sleep apnoea might enhance oxidative stress by reducing the antioxidant capacity of blood.9 The degree of oxidative stress in patients with OSA varies and is uncommon in patients with no comorbidities.4 Also, sex-related differences in susceptibility to oxidative stress have been noted; in oxidative brain injuries experimentally induced in mice, premenopausal females had greater protection than age-matched males or ovariectomised age-matched females.10 This sex-related susceptibility could be related to differences in cardiovascular morbidity associated with OSA.

Intermediate mechanisms linking OSA with cardiovascular disease The mechanisms for initiation and aggravation of cardiovascular disease have not been fully elucidated. Several pathogenic factors are proposed as intermediate mechanisms linking OSA with cardiovascular disease, mainly oxidative stress, sympathetic activation, inflammation, hypercoagulability, endothelial dysfunction, and metabolic dysregulation (figure). Although described here separately, these mechanisms are closely interrelated and manifest simultaneously in patients with OSA.

Published Online November 6, 2012 S2213-2600(12)70051-6 Respiratory Department, Hospital Universitari Arnau de Vilanova-Santa María, IRB Lleida, Lleida, Catalonia, Spain, and Centro de Investigación Biomédica en Red de Enfermedades Respiratorias (CIBERES), Madrid, Spain, (M Sánchez-de-la-Torre PhD, Prof F Barbé MD); and SleepDisordered Breathing Unit, Respiratory Department, Valme University Hospital, Seville, Spain (F Campos-Rodriguez MD) Correspondence to: Prof Ferran Barbé, Respiratory Department, Hospital Universitari Arnau de Vilanova-Santa María, IRB Lleida, University of Lleida, 25198 Lleida, Spain. [email protected]

Sympathetic activation Increased activity of the sympathetic nervous system constricts blood vessels and increases cardiac output. The degree of sympathetic nervous system activation correlates with the severity of the increase in blood pressure, and is Published online November 6, 2012






Sa02 Oxygen desaturation

Systemic hypertension Sympathetic activation

C3A2 C4A1

Endothelial dysfunction Hypercoagulability


Obstructive sleep apnoea

Heart failure


Inflammation Oxidative stress Metabolic dysregulation

Flow Chest Abdomen

Sudden death

Polysomnographic registration Intrathoracic pressure changes


Myocardial ischaemia and infarction

Figure: Obstructive sleep apnoea consequences and intermediate mechanisms that potentially contribute to risk of cardiovascular disease The events associated with collapse of the upper airway lead to brain arousal, intrathoracic pressure changes, and hypoxaemia and reoxygenation. Several intermediate mechanisms link obstructive sleep apnoea with the initiation and progression of cardiovascular diseases. SaO2=oxygen saturation. C3A2 and C4A1=electroencephalographic channels.

more pronounced in the context of metabolic diseases such as diabetes, obesity, and the metabolic syndrome.11 Measurement of sympathetic nerve activity in muscle yields accurate and direct information about sympathetic nerve impulses. Moreover, sympathetic tone can be evaluated by measurement of plasma and urinary catecholamines. There is evidence that sleep apnoea might be associated with increased sympathetic activity.12 Hypoxia and hypercapnia act synergistically to heighten sympathetic activity,13 and this effect is especially marked during the apnoeic event. Patients with sleep apnoea have a high frequency of arousals from sleep, which induce a sympathetic burst during each apnoeic episode. Patients with OSA have high sympathetic activity when awake, with further increases in blood pressure and sympathetic activity during sleep.12 Several randomised trials have shown that CPAP treatment reduces sympathetic nervous system activity and attenuates the increased sympathetic tone in patients with OSA.14–16

Inflammation Obstructive sleep apnoea seems to be associated with local and systemic inflammation. Local inflammatory changes could be partly due to snoring, which triggers vibration frequencies associated with soft-tissue damage. Histological analysis of tissues from patients with OSA undergoing uvulopalatopharyngoplasty shows substantial subepithelial oedema, excessive plasma cell infiltration, and reduction in the surface area of connective tissue papillae, which provide anchorage for the epithelium. Polymorphonuclear leucocytes are also increased in nasal lavage fluid of patients with OSA, compared with people without OSA.17 In addition to local inflammation, systemic inflammation is seen in patients with OSA. Chronic inflammation has been closely related to the formation 2

and progression of atherosclerosis. In patients with OSA, activation of redox-sensitive gene expression is suggested by an increase in specific protein products, including inflammatory cytokines; this implies the participation of redox-sensitive transcription factors, such as hypoxiainducible factor-1, activator protein-1, and nuclear factor κB (NF-κB). NF-κB is one of the key regulators of inflammation, immune response, and cell survival. A non-randomised study suggested that NF-κB is highly activated in patients with OSA compared with healthy controls, and that CPAP treatment reduces NF-κB activation.18 Several studies of patients with OSA have reported raised plasma concentrations of other inflammatory markers, such as cytokines (including interleukin-6 and tumour necrosis factor α), matrix metalloproteinases, and acute phase proteins, as well as endothelial adhesion molecules, such as intercellular adhesion molecule 1 and vascular cell adhesion molecules.19 Furthermore, adipokines are involved in a range of processes, including immunity and inflammation,20 and several studies have shown that patients with OSA have higher concentrations of proinflammatory adipokines than patients without OSA.21 These proinflammatory adipokines seem to contribute substantially to the inflammatory state of obese patients. Obesity is the most common comorbidity and is present in more than half of patients with OSA. Obesity is considered a chronic inflammatory situation in itself, therefore it might be the most important confounding factor in the association between sleep apnoea and inflammation. In fact, Guilleminault and coworkers22 reported that obesity had a strong association with C-reactive protein but not with OSA. A randomised trial by Kohler and colleagues23 reported no improvement in inflammatory markers after CPAP treatment. Published online November 6, 2012


Hypercoagulability There is evidence of a hypercoagulability status in patients with OSA, which might contribute to an increased risk of cardiovascular events. Patients with OSA show a morning increase in plasma fibrinogen concentration and whole blood viscosity, as well as low fibrinolytic activity.24 Furthermore, studies have reported high levels of coagulation factors XIIa, VIIa, and thrombin-antithrombin complex.25 Patients with OSA show increased platelet activation and aggregation, although the mechanisms of platelet activation are not completely understood and might be related to increased sympathetic activity.26 Hypoxaemia and repetitive arousals from sleep combine to produce raised concentrations of epinephrine and norepinephrine, and high numbers of circulating catecholamines cause concentration-dependent platelet activation in vitro and in vivo.27 Interventional studies have shown that CPAP treatment reduces coagulability and the risk of thrombosis in patients with OSA, although not all coagulability markers were reduced after CPAP treatment.28–31

Endothelial dysfunction The vascular endothelium is a confluent, cellular monolayer that lines the entire vascular compartment at the interface between blood and the vessel wall. The vascular endothelium is intimately involved in controlling vasomotor tone and is the main regulator of vascular haemostasis. The endothelium continuously adjusts the balance between vasoconstriction and vasodilatation; if this balance is tilted towards vasoconstriction, endothelial dysfunction occurs, causing damage to the arterial wall. Endothelial dysfunction occurs in response to cardiovascular risk factors and can precede or accelerate the development of atherosclerosis.32 Assessment of endothelial function in OSA involves functional evaluation of vascular responses—ie, recording changes in blood flow in response to endothelium-dependent vasodilators or hypoxaemia, quantification of levels of circulating apoptotic endothelial cells, and measuring plasma concentrations of various endothelial biomarkers.33 Several studies show indirect evidence of reduced nitric oxide availability and high plasma concentrations of adhesion molecules, suggesting that inflammation and vascular endothelial dysfunction contribute to the development of vascular diseases in patients with OSA.4 Moreover, increased sympathetic activation and oxidative stress, common in patients with OSA, might contribute to the development of endothelial dysfunction. Increased oxidative stress reduces nitric oxide availability and increases expression of ROS, which activates inflammatory pathways that facilitate the recruitment and accumulation of blood cells on the vasculature of the endothelial lining.3,4 Effective CPAP therapy (>4 h each night) in patients with OSA reverses vascular endothelial dysfunction and inflammation, and enhances endothelial repair capacity.34

Nguyen and colleagues35 showed that CPAP treatment improves microvascular disease and nitroglycerin-induced coronary vasodilation. CPAP withdrawal has also been associated with impaired endothelial function.36 Cross and coworkers37 noted that in patients with OSA, endothelial dysfunction is proportional to hypoxaemia and is improved by CPAP therapy. Therefore, evidence shows that OSA directly affects the vascular endothelium by promoting inflammation and oxidative stress, while decreasing nitric oxide availability and repair capacity. Furthermore, CPAP treatment reverses endothelial dysfunction and enhances endothelial repair capacity.

Metabolic dysregulation OSA-related factors such as increased sympathetic activity, sleep fragmentation, and intermittent hypoxia contribute to the development of metabolic dysregulation. Metabolic syndrome refers to a group of factors, including insulin resistance, dyslipidaemia, hypertension, and abdominal obesity, that together result in increased cardiovascular risk. Metabolic syndrome is common in patients with OSA, and OSA is frequently found in conditions associated with metabolic abnormalities.38 Studies show that patients with OSA have higher free fatty acid concentrations than controls, and this could be one of the mechanisms involved in the metabolic complications of OSA.39 Nevertheless, there is no clear metabolic pattern associated with OSA, and the single components of the metabolic syndrome found in patients with OSA vary. Independent of adiposity, OSA is associated with impairments in insulin sensitivity, glucose effectiveness, pancreatic B-cell function, and dyslipidaemia.40 There is much debate about whether OSA increases the risk of type 2 diabetes, and whether CPAP treatment could reverse insulin resistance. Phillips and coworkers16 showed that CPAP reduced post-prandial triglycerides and total cholesterol. Other clinical trials, however, have shown a partial benefit of CPAP treatment for metabolic components. Sharma and colleagues41 reported that 3 months of CPAP therapy partially reversed metabolic abnormalities. Hoyos and colleagues42 noted that 12 weeks of CPAP treatment did not improve insulin sensitivity, visceral abdominal fat, or liver fat, and Weinstock and collaborators43 showed that insulin glucose tolerance was not normalised after CPAP in patients with sleep apnoea and obesity. Since obesity often coexists with OSA, it is not yet clear whether the presence of metabolic disorders is a consequence of OSA or simply reflects the effects of coexisting severe obesity.44

Cardiovascular consequences of sleep apnoea Hypertension Hypertension is a common feature of patients with OSA. It is estimated that a third of hypertensive patients have OSA, and about half of patients with OSA are hypertensive.45 OSA is present in about 80% of patients with resistant hypertension, and is the leading recognisable cause of Published online November 6, 2012



hypertension in about two-thirds of these patients.46 The sympathetic activation associated with obstructive events during sleep might blunt nocturnal lowering of blood pressure and result in a higher occurrence of a nondipping pattern in patients with OSA. In patients with resistant hypertension, other pathogenic pathways might also be involved. Plasma aldosterone concentration is positively associated with the apnoea–hypopnoea index (AHI) in these patients, and OSA severity has been reported as being worse in patients with hyperaldosteronism. One explanation for these findings is that the fluid retention associated with hyperaldosteronism exacerbates OSA by increasing the movement of fluids to the surrounding tissues, particularly in the neck; this might contribute to the increased propensity for airway collapse during sleep. This hypothesis is supported by a study assessing rostral fluid shifts in patients with OSA and resistant hypertension, which showed that the severity of OSA is strongly related to the amount of fluid displaced from the legs overnight.47,48 Results from prospective longitudinal studies suggest that moderate to severe OSA is an independent risk factor for hypertension49 and for development of a nondipping blood pressure pattern.50 Two population-based studies have reported conflicting data. The Sleep Heart Health Study (SHHS)51 analysed 2470 individuals who were not hypertensive and who were followed up for 5 years. Although a dose–response relationship between AHI and risk of incident hypertension was identified, this association was not found after adjustment for bodymass index (BMI).51 The Vitoria Sleep Cohort,52 which included 1180 participants who were followed up for 7∙5 years, showed that after adjustment for age, sex, and BMI, there was no association between OSA and hypertension.52 These studies suggest that the association between OSA and hypertension might not be as strong as was once thought.

Effects of OSA treatment on hypertension Several meta-analyses53–55 have reported that CPAP treatment significantly decreases blood pressure by about 2 mm Hg, and that CPAP compliance and higher baseline blood pressure are predictors of successful blood pressure reduction (table 1). The main concerns with these early studies were the small number of patients enrolled, the presence of non-hypertensive patients, and the short follow-up period. Durán-Cantolla and coworkers56 did a study of 340 patients with recently diagnosed, untreated hypertension and moderate to severe OSA, who were randomly assigned to CPAP or sham CPAP and followed up for 3 months. Blood pressure was measured by 24-h ambulatory blood pressure monitoring (ABPM). The CPAP group showed a significant decrease in mean ABPM of 1∙5 mm Hg. Marin and coworkers69 did an observational, clinical cohort study in 1889 participants who were not hypertensive and were followed up for 12∙2 years. Compared with participants without OSA, 4

those with untreated OSA had a higher adjusted risk of incident hypertension; patients with OSA adequately treated with CPAP (at least 4 h per day) had a risk of hypertension similar to that of non-OSA participants. In a multicentre, randomised trial involving 359 hypertensive patients with OSA who were randomly assigned to CPAP or conservative treatment for 1 year, Barbé and coworkers57 showed that CPAP compliance of greater than 5∙6 h per night was associated with a reduction in blood pressure of about 3∙5 mm Hg. Theoretically, the effect of CPAP on blood pressure should be greater in patients with resistant hypertension; unfortunately, few studies have investigated this issue. In a randomised trial with 41 patients with ABPM-confirmed resistant hypertension, Lozano and coworkers58 showed a significant decrease in ABPM in patients who used CPAP more than 5∙8 h per day. Martinez-Garcia and coworkers59 randomly assigned 210 patients with resistant hypertension and an AHI higher than 15 to CPAP or usual treatment for 3 months. In the intentionto-treat analysis, CPAP treatment did not improve ABPM measurements. In a post-hoc analysis, patients with CPAP adherence above the average (>4 h per day) showed a significant reduction in 24-h mean systolic and diastolic blood pressure (5∙5 and 4∙2 mm Hg, respectively). Available evidence has led OSA to be considered an identifiable cause of hypertension.70 Overall, studies examining the effect of CPAP on blood pressure show a slight decrease of about 2 mm Hg. The magnitude of this decrease might be greater in patients with higher baseline blood pressure and higher compliance with CPAP treatment. However, in patients with OSA, the drop in blood pressure associated with CPAP treatment is much lower than the results obtained with antihypertensive drugs.71

OSA and non-fatal cardiovascular outcomes In a cross-sectional analysis from the SHHS (more than 6000 participants from the general population), participants in the upper AHI quartile were 42% more likely to report at least one cardiovascular disease, compared with those in the lower quartile.72 In a further longitudinal analysis of more than 4000 participants who were initially free of coronary heart disease and heart failure and were followed up for a median of 8∙7 years, the incidence of both conditions increased substantially with increasing OSA severity in men, although the increased incidence of coronary heart disease was only significant in men younger than 70 years.73 In an observational study, Marin and colleagues74 followed 1651 men for 10 years and showed that untreated, severe OSA significantly increased the risk of non-fatal cardiovascular outcomes, compared with healthy participants (odds ratio [OR] 3∙17, 95% CI 1∙12–7∙51; p=0·001). Patients with higher CPAP compliance (at least 4 h per day) reduced this risk to the level of non-OSA participants. So far, only one randomised trial has assessed Published online November 6, 2012


the long-term effects of CPAP on cardiovascular outcomes. Barbé and coworkers68 randomly assigned 725 consecutive non-sleepy (Epworth Sleepiness Scale score ≤10) patients with OSA to CPAP or no active intervention, and followed up participants for a mean of 4 years. No difference was found between CPAP-treated

and untreated patients with regard to incidence of a composite endpoint of hypertension or cardiovascular events (9∙20 vs 11∙02 per 100 person-years, respectively; table 1). However, the study had limited power to detect differences between groups. When the results were analysed on the basis of CPAP adherence, patients who



Patient population

Duration of follow-up

Key findings

Blood pressure

Therapeutic CPAP vs sham CPAP

340 patients with untreated hypertension and AHI at least 15/h

3 months

1·5 mm Hg decrease in mean ABPM, 2·1 mm Hg decrease in systolic ABPM, and 1·3 mm Hg decrease in diastolic ABPM

Barbé et al (2010)57 Blood pressure

Therapeutic CPAP vs conservative treatment

359 hypertensive patients with AHI at least 20/h, without daytime sleepiness (Epworth Sleepiness Scale score

Obstructive sleep apnoea and cardiovascular disease.

Obstructive sleep apnoea (OSA) is a common health concern caused by repeated episodes of collapse of the upper airway during sleep. The events associa...
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